In a typical known carburizing method, a workpiece is carburized in a carburizing atmosphere with a carbon potential equivalent to Acm transformation temperature or less, after raising temperature to the carburizing temperature range. Then, carbon is further diffused at the same temperature and with a carbon potential of 0.7 to 0.9 wt %. After temperature is lowered to about 850.degree. C., the workpiece is quenched. Alternatively, the workpiece once cooled subsequently to the prior diffusion process is heated again to about 850.degree. C. and then quenched (reheating hardening).
In recent years, high-temperature carburization is attempted, in which RX gas and butane gas are used as carrier gas and enriched gas respectively and such RX gas carburization is performed at a high temperature of 950.degree. C. to 1,000.degree. C. in order to increase the yield of carburized or carbo-nitrided steel. As other known high-temperature carburizing methods, the following carburization techniques are taken at high temperatures: (a) vacuum carburization in which carburization and diffusion are carried out in a reducing atmosphere in which hydrocarbon gas is decomposed at a reduced pressure; (b) N.sub.2 -base carburization in which carburization and diffusion are carried out in an atmosphere in which N.sub.2 gas mixed with hydrocarbon gas is heat-decomposed. The RX gas carburization method and N.sub.2 -base carburization method often use a continuous carburizing furnace to enable mass production. In the above methods, a target value of the carbon content at the surface of the carburized layer after carburization is such a value with which 0.7 to 0.9 wt % an eutectoid constituent can be achieved and, generally, there are precipitated no carbides on the surface of the carburized layer.
A special carburization technique, high-carbon carburization is also known, in which two or more carburizing cycles are repeated, at least one of which is carried out in an atmosphere with a carbon content equivalent to Acm transformation temperature or more so that carbides are dispersed in the surface layer of steel. This technique aims to increase the rolling strength of steel parts and an example of which is disclosed in Japanese Patent Publication (Kokoku) No. 62-24499 (1987). According to one embodiment of this publication in which RX gas carburization is incorporated, a workpiece is pre-carburized for 6 to 12 hours at a temperature ranging from 930 to 980.degree. C. with a carbon potential in the range of from the eutectoid carbon content to the value equivalent to Acm transformation temperature. After cooled by air or quenched once, the workpiece is again heated by raising temperature at a rate of 20.degree. C./min. or less to a re-carburizing temperature of 750 to 950.degree. C. Subsequently, carburization is carried out for 6 hours at 900.degree. C. while the carbon potential (=1.85) equivalent to Acm transformation temperature or more being maintained, whereby 30% by volume or more cementite is precipitated in the carburized surface layer of 0.1 mm. In addition to the description of the above carburizing technique, the publication has reported that the steel having 30% by volume or more cementite precipitates exhibits superior rolling life. In the high-carbon carburization which causes cementite diffusion, the steel workpieces are required to contain 0.5 wt % or more Cr in most cases, as disclosed in Japanese Patent Publication (Kokai) No. 6-17225 (1994).
Since a problem presented by the prior art RX gas carburization method lies in the carburization reaction based on CO--CO.sub.2 gas, there is inevitably created a grain boundary oxidized zone or imperfect hardened zone on the surface layer of steel after carburization, which results in, when taking a gear for example, decreases in the bending strength of the dedendums and in the strength of the tooth flanks. Due to the recent trend towards more compactness and higher load caused by transmitted power in reduction gears, there arise demands for a carburizing method which is able to prevent oxidization reaction under a carburizing atmosphere.
Carburization treatment in which the carbon content of the surface of a workpiece is controlled by controlling the amount of CO.sub.2 in a carburizing atmosphere is usually carried out at a temperature of 900 to 950.degree. C., because it is extremely difficult to control carbon potential by controlling the amount of CO.sub.2 under high-temperature carburizing conditions. This inevitably involves long processing time. For example, it is well known that treatment of large gears takes two or more days and therefore incurs very high treatment cost. In addition, prolonged carburization treatment often causes an increase in the depth of a grain boundary oxidized zone or imperfect hardened zone so that the tooth flanks must be ground in some cases, resulting in a further increase in the cost of manufacture of gears.
When performing the above RX gas carburization at high temperatures for instance, for adjusting carbon potential to 1.5.+-.0.1 during a carburization phase at a temperature of 1,000.degree. C., the amount of CO.sub.2 in the furnace must be controlled within the range of about 0.035 to 0.045%. For adjusting the carbon content of the surface to 0.8.+-.0.1 wt % during a diffusion phase, the amount of CO.sub.2 must be controlled within the range of 0.1.+-.0.02%. For adjusting the carbon content of the surface to 1.5.+-.0.1 wt % during carburization at a temperature of 1,100.degree. C., the amount of CO.sub.2 must be within the range of 0.015 to 0.020%, and for adjusting the carbon content of the surface to 0.8.+-.0.1 wt %, the amount of CO.sub.2 must be within the range of 0.035 to 0.05%. As understood from above, there are many problems in the control of carbon potential.
As attempts to solve the problems suffered by the above RX gas carburization method such as prolonged treatment and the creation of abnormal surface layers (e.g., a grain boundary oxidized zone), there have been proposed the vacuum carburization method and the N.sub.2 -base carburization method, in which carburization is performed in an atmosphere of hydrocarbon gas having a pressure of about 10 torr or less. However, the vacuum carburization method reveals the problem that even if the amount of CH.sub.4 within the furnace is measured and controlled, it cannot be used as an index for carbon potential and therefore CH.sub.4 needs to be added in an amount several times to several tens of times more than a theoretical value during carburization. This method thus fails in practically controlling carbon potential so that precipitation of coarsened cementite on the carburized surface layer cannot be prevented during carburization. Although some measures have been taken to avoid these undesirable results, for example, by stopping a supply of carburizing gas in the course of carburization to effect diffusion treatment for a specified period, these techniques are taken under various operating conditions which are determined according to the depth of carburization, the type of steel used and others, as described in the reference book written by Takeshi Naito. That is, the determination of operating conditions is highly dependent on the know-how, which leads to instability in quality, high maintenance cost for the system, and involvement of vacuum troubles. For adjusting the carbon content of the surface to 0.7 to 0.9 wt % in order to prevent the precipitation of platy pro-eutectoid cementite by cooling after carburization, the time required for diffusion should be, in general, two or three times the time required for carburization. This could be a chief factor for decreasing productivity particularly when a great depth of carburization is needed for instance in the case of large-sized gears.
The vacuum carburizing method suffers from the problem of soot produced by the decomposition of hydrocarbon gas. When treating a large number of parts, carburization under vacuum such as about 10 torr tends to entail carburization nonuniformity in the parts and therefore requires use of a larger amount of hydrocarbon gas, resulting in increased soot generation. Further, accumulating soot causes a lot of mechanical problems in performing continuous vacuum carburization.
With a view to preventing the grain boundary oxidation mentioned above, various N.sub.2 -base carburization techniques have been proposed. These techniques restrict oxidizing gas as much as possible but suffer from the same problem as that of the RX gas carburizing method in terms of carbon potential controllability, since the control of carbon potential during carburization in these techniques is performed by controlling the amount of CO.sub.2 gas.
Continuous carburization furnaces are often used for the purpose of increasing the productivity of the RX gas carburizing method and the N.sub.2 -base carburizing method. The continuous type is useful in dealing with a large number of parts under the same carburizing conditions but reveals ineffectiveness in the production of few-of-a kind parts which is prevailing in the recent trend. Even if continuous treatment is carried out with the vacuum carburization method, the same problem as noted above will be encountered in the production of few-of-a kind parts.
Coarsening of the crystal grains of austenite in steel during carburization is a common problem for all the high-temperature carburizing methods described above. It is anticipated that when such steel is used for producing a gear, the bending strength of the dedendums significantly decreases. Therefore, fining treatment is needed for the crystal grains of austenite, which is a disadvantage in adopting any of the high-temperature carburizing methods.
As a means for improving the strength of a rolling tooth face in carburization, there has been proposed the technique for dispersing cementite in an amount of 30 percent by volume or more, as noted earlier. Where the average grain size of cementite to be dispersed and precipitated is not precisely controlled like Japanese Patent Publication (Kokoku) No. 62-24499 (1987), the dispersed coarse cementite promotes the concentration of bending stress imposed on the dedendums and, in consequence, decreases the bending strength of the dedendums. This is a serious problem particularly in the technique of the publication in which carburization is carried out, precipitating carbides at a temperature of 950.degree. C. or less, while maintaining carbon potential at a value equivalent to Acm transformation temperature or more. In this technique, since the cementite at the outermost surface tends to be extremely coarsened and since carbides precipitate and aggregate in the grain boundary, not only the bending strength extremely deteriorates, but also the contact surface pressure strength is adversely affected. It is a particularly serious problem that decreases in contact surface pressure strength become more significant in high-speed rotating gears.
As described in the reference book written by Takeshi Naito, dispersion/precipitation of a carbide (cementite) in the carburized surface layer can be caused by repeatedly performing vacuum carburization cycles. In this case, cementite preferentially precipitates in the grain boundary like the other methods, and the cementite aggregates, creating coarse cementite grains which result in a considerable decrease in bending strength.
It is conceivable to add large amounts of alloying elements such as Cr or to lower re-carburization temperature in order to fine the precipitated cementite. However, the former case has the disadvantages that cementite precipitation occurs during the preceding high temperature carburization and that use of expensive steel is involved, whereas the latter case prolongs re-carburization time excessively, resulting in higher cost.
The present invention is directed to overcoming the foregoing problems and therefore aims to provide a method in which cementite grains fine enough not to substantially adversely affect fatigue strength are uniformly dispersed in the surface layer of steel and in which austenite crystal grains can be fined, this method being enabled by specifying the way of high temperature carburization and the constituents of steel to be used.
Further, the invention aims to provide a carburization system capable of providing high productivity and effectively dispersing finer cementite grains by employing carburizing methods improved over the above-discussed high temperature carburizing methods and re-carburizing methods.